When Jesus quotes his Bible as prohibiting false vows and other oaths (Deut 23:23), he probably also has in view the Ten Commandments, as in Matthew 5:21, 27. In this case he alludes to the third commandment: a false oath "misuses" or takes in vain God's name, since oaths by definition called on a deity to witness them (Ex 20:7). Breaking an oath was dangerous, for in all societies oaths contained curses that deities would avenge if the person who swore by them broke the oath. The Bible's point in prohibiting false oaths, however, was that one should tell the truth and keep one's promises. The Hebrew Bible approved of some oaths and vows (as in Num 5:19-22; 6:2), but Jesus again summons us beyond the law's letter to its intention. His own point is not so much that oaths are evil as that the motivation for engaging in them is; one should simply tell the truth (Mt 5:37).
Although Jesus' position on oaths is not wholly unique, it was rare enough to be distinctive. Although some Jewish teachers warned against customary oath-taking, nearly all accepted oath-taking as valid; in daily life, it was surely common in the marketplace. Some groups of Essenes may have avoided oaths altogether (Jos. War 2.135), except for their initiatory oath for joining the sect (Jos. War 2.139-42; see also 1QS 5.8). Josephus declares that one could trust an Essene's word more than an oath, however (War 2.135); Philo indicates that their abstention from oaths declared their commitment to truth (Every Good Man Free 84; also Vermes 1993:35). Jesus and the Essenes probably intended the same as Pythagoras: let your word carry such conviction that you need not call deities to witness (Diog. Laert. 8.1.22; compare Philo Spec. Leg. 2.2; Isoc. Nic. 22, Or. 2).
The point of this passage is integrity. Jesus observes that since God witnesses every word we say anyway, we should be able to tell the truth without having to call God to witness by a formal oath. Jesus is addressing a popular abuse of oaths in his day. To protect the sanctity of the divine name against inadvertent oath-breaking, common Jewish practice introduced surrogate objects by which to swear (Vermes 1993:34-35). Some people apparently thought it harmless to deceive if they swore oaths by something like their right hand (t. Nedarim 1:1; cf. Jos. War 2.451). The further removed the oath was from the actual name of God, the less danger they faced for violating it (Schiffman 1983:137-38; E. Sanders 1990:53-54). Jewish teachers had to arbitrate which oaths were actually binding as allusions to God's name (m. Sebi`it 4.13; see also CD 15.1-5). Jesus teaches that all oaths invoke God's witness equally. Just as heaven, earth (Is 66:1-2) and Jerusalem (Ps 48:2; Mt 4:5; 27:53) belong to God (Mt 5:34-35), so do the hairs on our heads (5:36); although we can dye our hair, we have no genuine control over its aging (compare 6:27). All oaths implicitly call God to witness, because everything that exists was made by him. For Jesus, no aspect of life except sin is purely secular.
Avoiding oaths is thus inadequate; the issue is telling the truth, because God witnesses every word we speak. Although many passages in the Bible allow some degree of deception to preserve life (Keener 1991a:22), such exceptions are rare in our daily lives. When we lie to cover our own wrong motives from those we think would disdain us, we forget that one day God will expose all the secrets of our hearts anyway (Mt 10:26). When we lightly commit ourselves to meet people at particular times and then unnecessarily delay them (as if their time were a commodity less precious than our own), we treat them unjustly and deceitfully, even if in a relatively minor way. How much more when we make promises in business deals or make still more lasting vows (such as the marriage covenant-5:31-32).
Making vows (promises) to God lightly is a severe offense (compare Acts 5:1-11). Although Jesus' first followers continued to call on God to witness the truth of some of their statements, apparently taking Jesus' words as rhetorical overstatement (examples appear in Rom 1:9; 9:1; Gal 1:20), they seem to have refrained from more overt oaths (2 Cor 1:17; Jas 5:12). Oaths that invite penalties on oneself for violating them ("cross my heart and hope to die") are unnecessary for people of truth.